We map STG registers onto appropriate CmmExprs. Either they map
to real machine registers or stored as offsets from BaseReg. Given
a GlobalReg, get_GlobalReg_addr always produces the
register table address for it.

Here is where the STG register map is defined for each target arch.
The order matters (for the llvm backend anyway)! We must make sure to
maintain the order here with the order used in the LLVM calling conventions.
Note that also, this isn't all registers, just the ones that are currently
possbily mapped to real registers.